The present invention is related to an inspection apparatus for inspecting circuit patterns of semiconductor devices formed on semiconductor wafers, circuit patterns of photomasks formed thereon, and the like.
As methods for inspecting defects which are present in circuit patterns formed on substrates, or boards to be inspected, e.g., semiconductor wafers and photomasks, inspection apparatus using electron beams and capable of accepting very fine processing of circuit patterns are known in this field in addition to inspection apparatus using optical images.
This inspection apparatus is designed in such a manner that SEM (Scanning Electron Microscope) is applied thereto, by which an image is produced from secondary electrons and reflected electrons, which are generated by irradiating an electron beam onto a substrate to be inspected. This inspection apparatus compares SEM images as to the same circuit pattern with each other so as to extract different pixels from the SEM image, and thus, recognizes these different pixels as defects.
In this inspection apparatus, image qualities of acquired SEM images are different from each other, depending upon a total number of irradiating operations of the electron beams with respect to the substrate to be inspected. When a total number of the electron beam irradiating operations is small, since an information amount as to an acquired inspection region is small, such a technique is known from, for example, JP-A-5-258703. That is, the same inspection region is irradiated plural times in order to acquire a sufficiently large number of images for an inspection purpose.
Otherwise, when a total number of the electron beam irradiating operations is large, since irradiation energy is high, there is such a problem that image contrast is changed due to a charging effect of the substrate to be inspected. As a result, another technique capable of acquiring an image by irradiating an electron beam one time is known from, for example, JP-A-2000-193594.
In this inspection apparatus, in such a case that an image is acquired by irradiating the electron beam one time, the following problems may occur due to a property of a substrate to be inspected. That is, contrast of the acquired image is fluctuated, and also, sufficiently higher contrast as to a subject portion cannot be obtained, since the electron beam is irradiated one time. To solve the above-explained problems, one solving method is performed in such a manner that since the same region of the image is irradiated by the electron beams plural times, an irradiation energy amount of the electron beams is increased. In this case, it is so assumed that a single transport of the substrate to be inspected which is transported along one direction while being scanned by an electron beam is referred to as a “line.” Thus, in the case of a single irradiating operation, after a scanning operation of 1 line is accomplished, the substrate to be inspected is transported by 1 line along a direction perpendicular to the line, and then, an adjoining line is irradiated by an electron beam, whereas in the case of a plural irradiating operation, 1 line is irradiated plural times without transporting the substrate to be inspected along the perpendicular direction.
However, if the irradiation energy amount is increased in such a method, then there is such a risk. That is, a potential is locally and temporarily changed, so that a semiconductor wafer corresponding to the substrate to be inspected is destroyed. Also, since the same line is irradiated plural times, inspection time is delayed.